I haven't updated this project log for a while, but I have been working on a few things. One thing that I just finished is the design for the brackets to hold the fans themselves. I had a couple iterations of this, but I eventually simplified it to make it less confusing. I designed it in OpenSCAD so that it's easy to change dimensions. The SCAD file is part of the design repository. This image of the reference design (the "reference" preset) has absurd dimensions, but I did it that way so that the labels for the parts would be very visible and in unambiguous locations.

Here's a photo of a more realistically-dimensioned "reference" bracket (the battery is just for scale):

The bracket design incorporates the following ideas:

• Of course, you can customize any of the dimensions.
• The inner box (the "fan box") at the bottom holds two fans side by side. It's not visible in the above images, but there are holes in the bracket for screws.
• The bracket can either hang from the sides of the vent opening or can stand on the floor of the vent cavity.
• Accordingly, the horizontal tabs (at the top of the images) can be set to point inward or outward.
• The vertical risers can attach to the fan box right at the fan box ends or offset inward or outward.
• There is an optional cross-brace near the middle in the images, for added stability. There are holes in the cross-brace so that you can put a screwdriver through to get at the screws on that side of the fan box.

Here's what it looks like with 3 fans (92mm) held together with 4 brackets. (I didn't use a cross-brace.) The brackets on the end are not strictly necessary, but I used them for added stability.

I printed the brackets with PETG filament. It took a little trial and error to dial in the dimensions.